Transcript Cohen_Gagne_theta1OriC_MCWS_slides_v3

Agreement between X-ray data and
magnetically channeled wind shock
model of q1 Ori C
David Cohen, Marc Gagné
for Massive Star research group
v.2 December 24, 2007
context
Gagné et al., 2005, ApJ, 628, 986 (and the erratum:
2005, ApJ, 634, 712) discusses four separate Chandra
grating spectra, covering different rotational phases of
the star. A coherent picture of magnetically channeled,
shock heated magnetospheric plasma is presented,
integrating several aspects of the X-ray data with UV, Halpha, and magnetic field measurements.
2-D MHD modeling of MCWS (by Asif ud-Doula) is
presented in this paper. The overall agreement
between these models and the data is very good; and
the detailed dynamical models confirm much of the
qualitative physical picture of the magnetospheric
properties.
outline
Context: we would like individual annotated images
showing the agreement between the x-ray data and the
MCWS modeling, for group members to use in their
presentations. The points of agreement include:
phase-dependence of the x-ray flux
forbidden-to-intercombination line ratios
temperature distribution (DEM)
line widths
Central 7 arcminutes of the Orion Nebula
Cluster as seen by Chandra ACIS-I
arrow pointing at q1 Ori C
Red: < 1.5 keV
Green: 1.5 keV < E <
2.5 keV
Blue: > 2.5 keV
Note: there is no diffuse x-ray emission; you’re just seeing the wings of the point spread function;
and the black dot at the center of the source is an effect of pileup in the ACIS detector.
Dipole magnetic field
(> 1 kG) measured on
q1 Ori C
Wade et al. (2006)
Magnetic field obliquity,
 ~ 45o, inclination, i ~ 45o
Predictions from the MCWS model and MHD
simulations:
Bulk of hot plasma is in the closed field region
(< Alfven radius; h(r) > 1) ~2R* for q1 Ori C
Significant shock heating – plasma very hot
(few 107 K)
Post-shock plasma moving quite slowly
Four separate diagnostics address these three
predicted properties
The phase dependence of the x-ray flux
constrains the spatial distribution of the x-ray
emitting plasma in the magnetosphere.
A deeper eclipse at edge-on viewing angles if the
hot plasma is closer to the star.
Fortuitous access to all viewing angles of the
magnetic field
Cartoon showing viewing angles of θ1
Ori C for Chandra observations in
Gagné et al. (2005). Phase 0 is when
the disk is viewed face-on (α=4 deg),
while phase 0.5 occurs when the disk
is viewed edge-on (α=87 deg)
Gagné et al. (2005)
Rotational modulation of the X-ray emission from
variation in the occultation of the x-ray emitting
magnetosphere by the star
To 1st order, the depth of eclipse depends on how
close the shock-heated plasma is to the star
Chandra broadband count rate vs. rotational phase
0.4
0.3
1.0
0.2
0.5
0.1
0.0
0.0
0.2
0.4
0.6
0.8
Rotational phase (P=15.422 days)
Model from MHD simulation
0.0
1.0
Simulation EM (1056 cm-3)
θ1 Ori C ACIS-I count rate (s-1)
1.5
Note that the assumption in the last two figures
is that the only thing that modulates the x-ray
flux is occultation of the magnetosphere by the
star itself.
Fig. 14 and Table 6 in Gagné et al. (2005)
indicate that there might be a modest excess
of absorbing column when the
magnetosphere is viewed edge-on.
The ratio of the intensity of the helium-like
forbidden to intercombination lines also constrains
the spatial distribution of the x-ray emitting plasma
in the magnetosphere.
A weaker forbidden line indicates the plasma is
closer to the star, since the upper level of the
forbidden line is depopulated by the local UV
radiation field.
Helium-like ions (e.g. O+6, Ne+8, Mg+10, Si+12, S+14) –
schematic energy level diagram
1s2p 1P
10-20 eV
1s2p 3P
1s2s 3S
resonance (r)
1-2 keV
forbidden (f)
intercombination (i)
g.s. 1s2 1S
The upper level of the forbidden line is very long lived –
metastable (the transition is dipole-forbidden)
1s2p 1P
10-20 eV
1s2p 3P
1s2s 3S
resonance (r)
1-2 keV
forbidden (f)
intercombination (i)
g.s. 1s2 1S
While an electron is sitting in the metastable 3S level, an ultraviolet
photon from the star’s photosphere can excite it to the 3P level – this
decreases the intensity of the forbidden line and increases the
intensity of the intercombination line.
UV
1s2p 1P
1s2p 3P
1s2s 3S
resonance (r)
forbidden (f)
intercombination (i)
g.s. 1s2 1S
The f/i ratio is thus a diagnostic of the strength of the
local UV radiation field.
UV
1s2p 1P
1s2p 3P
1s2s 3S
resonance (r)
forbidden (f)
intercombination (i)
g.s. 1s2 1S
If you know the UV intensity emitted from the star’s surface, it
thus becomes a diagnostic of the distance that the x-ray
emitting plasma is from the star’s surface.
UV
1s2p 1P
1s2p 3P
1s2s 3S
resonance (r)
forbidden (f)
intercombination (i)
g.s. 1s2 1S
Model of f/i ratio dependence on the radius (via the
dilution factor)
model
constraints from
data
implied constraints on location
helium-like magnesium
Mg XI
in q1 Ori C
R
I
F
single source radius
assumed
data constrain:
1.0 R* < Rfir < 2.1 R*
Rfir=1.2 R*
Rfir=2.1 R*
Rfir=4 R*
Temperature distribution in the X-ray emitting
plasma is predicted to be skewed toward high
temperatures, due to the strong, head-on shocks
near the magnetic equator.
The temperature distribution (‘differential emission
measure’) is derived from fitting thermal
equilibrium models (e.g. APEC) to the dispersed
spectrum.
Differential emission measure
(temperature distribution)
MHD simulation of q1 Ori C
reproduces the observed
differential emission measure
Wojdowski & Schulz (2005)
Emission line widths trace the line-of-sight velocity
distribution in the hot plasma.
The magnetic confinement of the post-shock
plasma predicts small, but not infinitely narrow,
line widths.
Line profiles: resolved,
but narrow
q1 Ori C: Ne X
Ly-alpha
z Pup
(O4 If)
Normal O star, with
a much broader
emission line, for
comparison.
Distribution of X-ray line widths in q1 Ori C
cooler lines: broad
(LDI wind shocks)
hotter lines: narrow,
but resolved
Note that the UV wind lines in q1 Ori C imply a wind terminal
velocity variously reported as ~1500 to in excess of 2500 km/s.
Gagné et al. (2005)
EM per unit volume (1110 ks)
MHD sims:
5
z-axis (stellar
radii)
HWHM ~
200 km/s
0
No viewing
angle
dependence
-5
-5
1x1055
0
5
x-axis (stellar
radii)
Line profile
(1110 ks) – tilt: 0 deg
1x1055
6x1054
4x1054
2x1054
0
deg
8x1054
EM (cm-3)
EM (cm-3)
8x1054
Line profile (1110 ks) – tilt: 90
6x1054
4x1054
2x1054
-500
0
500
Line-of-Sight Velocity (km/s)
0
-500
0
500
Line-of-Sight Velocity (km/s)
The following slides demo the
basic results from the MHD sims
MHD simulations of magnetically channeled wind
temperature
emission measure
simulations by A. ud-Doula; Gagné et al. (2005)
Channeled collision is close to head-on –
at 1000+ km s-1 : T = 107+ K
Emission measure
contour encloses T > 106 K
MHD simulations show multi-106 K plasma,
moving slowly, ~1R* above photosphere
contour encloses T > 106 K
The following slides show the Chandra grating
(MEG) data, with that of z Pup for comparison.
The helium-like complexes of several ions
are visible;
The high temperatures are apparent in the
line ratios (and strong continuum);
The modest line widths are easily seen when
compared to z Pup’s.
Chandra grating spectra (R ~ 1000 ~ 300 km s-1)
q1 Ori C
q1 Ori C: hotter plasma, narrower emission lines
z Pup
z Pup (O4 I): cooler plasma, broad emission lines
H-like/He-like ratio is temperature sensitive
q1 Ori C
Si XIV
Mg XII
Si XIII
Mg XI
z Pup
The young O star – q1 Ori C – is hotter
q1 Ori C
Si XIV
Mg XII
Si XIII
Mg XI
z Pup